This invention relates generally to digital communication systems and more particularly to the generation of soft quantized data values representative of the error probability of the received binary data in a digital receiver employing non-linear demodulation.
Digital communication systems, such as mobile data systems, are employing increasingly complex coding schemes to reduce the effect of channel noise. Decoders employing soft quantization are attractive for improving performance, but typical frequency shift keying (FSK) detectors used in mobile data systems are non-linear. The detected binary data is therefore not representative of the error probability of the detected data since the detector transforms the received data to binary ones or zeros, i.e. hard quantized data. FM threshold and limiting effects therefore ordinarily prevent good resolution of soft quantized data values from the signal level of the FSK detector output.
To solve this problem a soft quantizer utilizing an analog signal level derived from the audio noise level of the FM discriminator output is disclosed. This analog signal level is a better indication of the magnitude, or numerical weight, of the detected binary bits than the hard quantized data bits from the FSK detector output, so that the benefits of soft quantization in the decoding process can be obtained for FM/FSK mobile data systems.